A clock with a constellation display has been known for displaying constellations in accordance with a time. The prior art constellation display clock has an hour hand, a minute hand, and possibly a second hand at a rotational center of the celestial sphere and a constellation disc rotates in accordance with the movement of these hands to display constellations visible at that instant.
However, since the hands representing time radially extend from the rotational center of the celestial sphere in the prior art constellation display clock, fixed stars, particularly those in the vicinity of the center of the celestial sphere, for example, the Pole Star is concealed by a rotating central portion of the hand. Accordingly, this is not satisfactory as a constellation disc.
Since the prior art constellation display clock has no backlight or one with insufficient illumination effect, a uniform luminous intensity cannot be obtained all over the constellation disc.
In the prior art, there is no constellation display clock having a function for representing a solar time as well as a sidereal time, in addition to a function for displaying constellations. On the other hand, in a prior art clock having a function for representing a solar time and a sidereal time but without a constellation disc, the solar time and the sidereal time are operated by individual oscillation circuits, respectively. This is because, while the solar time has a slight difference from the sidereal time (the sidereal time is shorter than the solar time, i.e., passes faster, by about 4 minutes per day), a time ratio between the solar time and the sidereal time cannot be approximated at a higher accuracy by a simple integer ratio, and because the sidereal time clock is mainly utilized for a measurement use in which even a small error caused by the use of the common single oscillator cannot be permitted.
In the prior art constellation display clock, it is impossible to solely control and rotate a constellation disc because the constellation disc is driven by a torque from the clock. Also the clock has a structure by which the positional adjustment of the constellation disc itself can be carried out only manually. A deviation between the constellation disc and the sidereal time is manually corrected.
As stated above, since a prior art clock having a function for representing solar time and sidereal time requires two oscillators and circuits, a control system thereof becomes complicated and expensive. In addition, since the control of the constellation disc independent from other parts is impossible in the prior art constellation display clock, various functions of the clock accompanying the rotation of the constellation disc are not satisfactorily achieved.
An object of the present invention is to solve the above problems inherent to the prior art and to provide a constellation display clock having no hands at the center of the constellation disc and having a simple driving means for accurately moving the constellation disc in accordance with the passage of time.
Another object of the present invention is to provide a constellation disc having a backlight on the back side of a constellation disc whereby the light emitted therefrom substantially homogeneously illuminates all over the constellation disc.
A further object of the present invention is to provide a constellation display clock which represents a solar time and a sidereal time at an accuracy sufficient for the practical use of the constellation display clock while using a single oscillator.
A further object of the present invention is to provide a constellation display clock having, in addition to a function for always displaying an accurate position of the constellations while corresponding the constellation disc with a sidereal time, various functions for rotating the constellation disc by independently controlling the rotation of the constellation disc while counting sidereal time and the position of the constellation disc.